In recent years, Friend, R. H., et al. have disclosed an organic EL device having a emitting layer comprising a polyphenylene vinylene (PPV) as a emitting material (International Disclosure WO90/13148). This organic EL device is meant to take the electroluminescence effect by the injection of carriers (holes and electrons) from appropriate injecting electrodes into the conjugated polymer. The inventors have enumerated the characteristic features of the organic EL device as follows: (1) it is stable to oxygen and moisture, unsusceptible to changes even at high temperatures, (2) the emitting layer is adhesive with respect to an underlying electrode and the like, having resistance to cracks of thermal or mechanical origin, (3) the PPV is not liable to recrystallize and (4) the PPV is highly crystallizable and has a high melting point, preventing the migration of ions or atoms.
However, this organic EL device comprises a PPV as a completely conjugated polymer hardly capable of emitting in blue and having a low fluorescence yield. Friend, R. H., et al. has made clear that the above shortcoming is explained by the low fluorescence yield of the PPV (J. Phys. D20, 1367 (1987) and J. Mol. Electronics; 5, 19, (1989)). Said PPV has the fluorescence yield of 1% or less. The electroluminescence efficiency (hereinafter referred to as EL efficiency) is lower than the fluorescence yield, and sufficiently high brightness cannot be obtained since the fluorescence yield is as low as 1% or less. Furthermore, thin films of PPV can be formed simply by spin-coating a soluble precursor, and heat-treating the above precursor film. But the optimum conditions of this heat treatment are not an easy matter to determine, and are liable to permit the conjugation of the main chain to have defects causing a decrease of EL efficiency. Thus it is difficult or entirely impossible to use a thin film of a conjugated polymer such as polyphenylene vinylene as the emitting layer in an organic EL device.
Furthermore, D. Braun et al. have disclosed a device having a emitting layer comprising a thin film which is composed of spin coated soluble PPV derivative with a long chain alkoxy substituent group (Appl. Phys. Lett. 58, 1982 (1991)). This PPV derivative also has a low fluorescence yield (quantum yield) and thus the EL quantum yield is as low as 5.times.10.sup.-4 if an in cathode is used and 10.sup.-2 if a Ca cathode is used. The color of the EL due to this PPV derivative is limited to orange.
Furthermore, Hosokawa et al. have disclosed an organic EL device having a emitting layer comprising a low molecular weight molecule with an arylene vinylene structure (EP No.0373582 and EP No. 0388768, etc.). This organic EL device is capable of emitting EL in greenish blue at a brightness as high as 300 cd/m.sup.2 and having a luminescence efficiency of 2.9 lumen/W even with the application of a meager 5V. They also have invented emitting layers made of substances having the similar structures which are capable of not only emitting EL in blue at a high brightness over 1000 cd/m.sup.2 but also emitting EL in a variety of colors range from bluish purple to green in sufficiently high brightness and efficiency.
These emitting layers are formed by the vapor deposition method. However, the vapor deposition method is more time consuming and less-productive than the spin coating method. Meanwhile, the vapor deposition films of an organic low molecular weight substance are liable to recrystallize on account of changes with the elapse of time or heat generation incidental to the operations. The recrystallized films no longer work as a emitting layer or as a hole injecting layer. The ability to retain thin films is missing in products of the vapor deposition method.
Under the circumstances, the present inventors have made extensive studies with a view to overcoming these shortcomings of the prior arts and developing an organic EL device comprising a emitting layer of a thin film easy to be formed by the spin coating method and capable of emitting EL in the color range from blue to green in sufficiently high brightness even on an application of a low applied voltage.